Honour - Usual dust buildup from distributing to usual* will be stuck in the Distribution module

[sherlock-admin2]

Honour

Medium

Usual dust buildup from distributing to usual* will be stuck in the Distribution module

Summary

The amount of Usual tokens minted by the distributionModule to distribute to UsualSP will always be slightly higher than the actual amount of Usual token taken by the UsualSP contract. The reason for this is that UualSP adjusts the amount before transferring the tokens from the distribution module.
Hence , there'll be a dust buildup of minted Usual tokens in the distribution module.

Root Cause

In RewardAccrualBase::_startRewardDistribution we see that the rewardAmount is adjusted (i.e. adjustedAmount = amount / duration * duration) before transferring the tokens into the contract. Due to this adjustment not all the Usual tokens minted by the distributionModule will be distributed by UsualSP.
The Usual token is a standard ERC20 so the dust(rewardAmount - adjustedAmount) will be little for a single ddistribution, however, given the distribution frequency(daily) this amount can accumulate over time and will be stuck in the distribution module.

Internal pre-conditions

No response

External pre-conditions

No response

Attack Path

No response

Impact

Medium - dust buildup of Usual tokens will be stuck in the distribution module

PoC

No response

Mitigation

1. Amount should be adjusted in the DistributionModule in _distributeToUsualStarBucket before minting the Usual tokens
2. There can be a sweep functionality to sweep the dust.
3. After minting the tokens startRewardDistribution can be called with usual.balanceOf(address(this)) as the rewardAmount to account for previously undistributed tokens. This approach will also distribute donations though